A. Field of the Invention
The present invention relates to an organic EL panel, in particular, an organic EL panel performing segment display in which the contrast between bright and dark states of the panel is improved.
B. Description of the Related Art
Since C. W. Tang et al. of Eastman Kodak Company announced an organic EL panel having a two layer structure and exhibiting high efficiency in 1987, various types of organic EL devices have been developed, and some of them began to be practically applied (C. W. Tang and S. A. van Slike, Appl. Phys. Lett. 51, 913 (1987)).
Methods for full color display have been studied: a device with plural types of organic EL light emitting elements each type emitting different color light are arranged on a substrate (so-called patterned RGB method); a device using a color conversion method, in which wavelength distribution of a back light is converted (hereinafter referred to as “CCM method”); and a device using a color filter method, in which light emission from a back light is irradiated through color filters. A color conversion layer and a color filter layer can be deposited without metallic mask and patterned as desired by means of a photo process. Therefore, the CCM method and the color filter method are advantageous for achieving a large area and high definition display.
Color panels capable of multicolor display are being developed today in organic EL panels. In some cases using a color panel there can be a section (for example an icon section or a time indicating section) in a panel that needs continuous lighting. However, the continuously lit section, in which current is running continuously, may cause problems of image sticking or degradation of brightness. To address this problem, a means is occasionally employed in which the continuously lit section is a segment displayed by static driving instead of by matrix driving. An organic EL panel 500 can be formed as shown in FIG. 1, comprising variable display section 600 that is matrix driven and segment display sections 650 that are statically driven.
In a segment display section formed using a CCM method or a color filter method, if the whole segment display section is composed of a single CCM or color filter, the opening becomes large. External light incident on the large opening is reflected from the reflective electrode and irradiated outwardly. As a result, the segment display section looks like it is emitting light even in the dark state (in the unlighted period), causing a problem of decrease of the contrast ratio between bright and dark periods (see FIG. 2). In the structure of FIG. 2, a color modulating section 20 composed of a color filter and a color conversion layer is formed on substrate 10. An organic EL device comprising transparent electrode 60, organic EL layer 80, and reflective electrode 90 is provided over overcoat layer 40 and passivation layer 50. Of external light 100 incident to the substrate, the component transmitting the color filter (that is light 110 in a color to be displayed by the segment display section) transmits through the color conversion layer and the subsequent layers, is reflected from the reflective electrode, transmits again through the color filter, and emits outwardly (reflection light 120). In addition, scattering light 130 scattered in the layers through which light 110 transmits is also emitted. Reflection light 120 and scattering light 130 have the wavelength component transmitted through the color filter. This means that, of incident light 100, only the light in the color to be displayed by the segment display is emitted. As a result, the contrast ratio decreases.
To address the degradation of contrast ratio, it has been proposed to paste a circular polarization film on the panel surface (Japanese Unexamined Patent Application Publication No. 2003-144830). However, the circular polarization film, though effective for reducing reflected light, cuts a part of the light emitted from the organic EL device in the bright period. So, the circular polarization film has a drawback causing degradation of emission efficiency of the panel as a whole. The decrease of emission efficiency is unfavorable because it results in increased power consumption, and a shortening of degradation lifetime of brightness of the organic EL device.
A method is therefore demanded in a segment display section employing a CCM method or a color filter method that prevents decrease of the contrast ratio due to reflection of external light and decrease of the emission efficiency in the bright period, which occurs in the use of a circular polarization film. The present invention is directed to overcoming or at least reducing the effects of one or more of the problems set forth above.